Exposure control system for a mass spectrometer responsive to the ion beam intensity



TOMIYOSHI OHTA 3,370,171

Feb. 20, 19%

EXPOSURE CONTROL SYSTEM FOR A MASS SPECTROMETER RESPONSIVE TO THE ION BEAM INTENSITY Filed Jan. 4, 1965 vibrating reed eLect rometer IN VENTOR TOMIYOSHI OHTA ATTORNEYS EXPOSURE CONTROL SYSTEM FOR A MASS SPECTROMETER RESPONSIVE TO THE ION BEAM INTENSITY Tomiyoshi Ohta, Tokorozawa-shi, Japan, assignor to Nihon Denshi Kabushiki Kaisha, Tokyo, Japan, a corporation of Japan Filed Jan. 4, 1965, Ser. No. 422,893 9 Claims. (Cl. 250-419) ABSTRACT OF THE DISCLOSURE A control system in a mass spectograph utilizing a photosensitive plate having a shielding member formed of an electrically conductive material and having an opening, the dimension of which is smaller than the cross section of the beam and which is positioned within the path of the beam to interrupt its flow and allow only a portion thereof to pass through and penetrate the photosensitive plate. The balance of the beam impinges upon the electrically conductive material so as to impose an electric voltage therein. The impinged voltage is built-up and accumulated in a condenser or capacitor, and an electrometer measures the buildup in the condenser and interrupts the electric current of the ion source when said potential has reached a predetermined voltage.

This invention relates to an improved exposure control system for a mass spectrometer.

In conventional mass spectroscopy, an unknown sample such as a metallic, inorganic and/ or organic substance is analyzed by observing its spectrum on a photographic plate. The exposure occurring on the photographic plate must be selected for each determination in accordance with the brightness of impingement thereon. To eliminate the difliculties encountered in making such selection, conventional apparatus is provided with a shutter. Electromagnetic and electrostatic shutters have been employed to control the ratio of brightness to time.

The electromagnetic shutter operates on the principle of shielding a small aperture through which the ion current passes. The apparatus employed for such a system requires construction tolerances in the order of microns that are difiicult to attain, and, additionally, the use of such apparatus raises questions of error in timing. Further, every time this slit shutter system is operated, the inside of the spectrometer is unnecessarily contaminated by scattered ions from the continuous ion source. The electrostatic shutter has merit in that it provides a high response that differs from a mechanical shuttering movement. However, the same undesirable scattered ion contamina'tion occurs as in the use of the eletcromagnetic shutter because the ions are merely deflected so that they do not reach the photographic plate and accordingly scatter within the apparatus.

The system of the present invention overcomes these difficulties and provides a very effective and novel method to control the exposure on a photographic plate without adverse contamination.

In the drawing, I have illustrated a preferred embodiment of my invention in which:

FIGURE 1 shows an illustrative plan view of a mass States Patent 3,3 76,171 Patented Feb. 20, 1968 spectrometer which demonstrates the principles of the present invention; and

FIGURE 2 shows an illustrative plan view of an embodiment of the present invention.

In FIG. 1 ionized molecules of a sample 6 attached to the top of a cathode 5 in a spark ion source 2 enter into the analysis column 1 after being accelerated by a main slit member 7. A slit member 8 which is positioned behind the main slit member 7 serves to shield the ion beam 3. As a result, the electric charge of the ion beam which is shut out by the slit member 8 is conducted to and stored in a condenser 9. The stored electricity in the condenser 9 is proportional to the quantity of ions which reached the surface of the photographic plate 4. Consequently, the exposure rate on the photographic plate can be read by determining the electric charge stored in the condenser 9 and this is accomplished by means of an electric charge meter 10. A relay 11 operates when the stored electric charge in condenser 9 reaches a fixed degree, so that if a relay value is properly predtermined, it is possible to control the electrical current through the relay by opening or shutting switch 11a which leads to the ion beam source of electrical current.

The numbers in FIG. 2 that correspond to numbers in FIG. 1 identify members that accomplish the same or a corresponding function in the two embodiments.

An oscillation tube 12 in combination with elements such as coil 13 and condenser 14 form an oscillation circuit shown generally at 15. The oscillation circuit 15 is disposed to provide a high frequency current, such as 1 mc., through the coil 13 to the grid of the power tube 23.

Although a high frequency of 1 me. is supplied through the coil 13 to the grid of power tube 23, the bias imposed by battery 16 prevents oscillations from circuit 15 from reaching tube 23.

In a thyratron 17 is not conductive, an astable multivibrator circuit 20 which includes tubes 18 and 19 provides periodic pulse oscillations. These pulse oscillations are amplified to a considerable extent by a tube 21. The circuit which includes tube 21 is constructed in such a manner that a positive pulse current is provided to the secondary side of transformer 22. The resulting pulse output is fed through the coil 13 to the grid of tube 23. The pulse oscillations from multivibrator 20 are necessary to trigger oscillation in oscillator 15 so that oscillator 15 will provide high frequency current to the grid of tube 23. Tube 23 is constructed in such a manner that it operates or discharges pulsating current only by such positive pulse input. That is, these currents comprise the high frequency oscillation circuit.

The high frequency pulse supply is additionally stepped up by a transformer 24 and supplied to the cathode 5 on which sample 6 is mounted. Cathode 5 is provided with additional accelerating current by battery 27. As a result of this circuit an appropriate spark discharge is attained.

As in the embodiment of FIG. 1, the ion beam is accelerated as it enters into the analysis column. This current passes through an electrostatic field 25 prior to reaching the first slit member 26. The ion current is shielded by the first slit which has an aperture with a fixed Width and height and, by determining the shielded ion current with an ammeter (not shown), the entire ion current can be determined. An additional slit member 8 contains a slit of about half the aperture size of the slit of member 26 and, accordingly, half of the ion current passing through the aperture of the first slit reaches the photographic plate 4 to provide the spectrum of sample 6. The rest of the ion current shielded by the slit of member 8 is conducted through one of the condensers shown generally at 9 via switch 28. If switch 29 is open and switch 30 is closed (as shown), an electric charge stored in one of the condensers 9 is determined by a vibrating reed electrometer 10. This meter is constructed in such a manner that, when a predetermined electric charge is stored in the operating condensers 9, the instrument gives an appropriate voltage such as 1 volt to the grid of thyratron 17 and cause current to flow to the plate of this tube. Current flow in tube 17 increases the voltage supplied through resistor 31 that fiows to the grid of tube 18, and such current flow interrupts the pulsations of the multivibrator 20. Thus, the grid of tube 23 is cut off from the positive high frequency current and the ion source 2 is simultaneously deenergized. Thus, the amount of ions reaching the photographic plate 4 is determined by the charge stored in the utilized condenser of condensers 9.

At the conclusion of the analyses, switch 29 may be closed to discharge the electric charge stored in the appropriate condensers of condensers 9. Thyratron 17 continues to operate due to the characteristics of such a tube so that no sparking or ionization takes place.

If switch 30 is now open, while switch 29 is also open, the thyratron 17 is interrupted and stops so that the multivibrator is activated and the circuit once again provides a spark and ionization in the spectrometer.

A fixed value of electric charge in condensers 9 can be varied by the capacitance of condensers 9 9 9 The meter may be of a type to measure the preselected charge directly from slit member 8 in which event condensers 9 and switches 28 and 29 may be eliminated from the circuit.

As mentioned above, in the preferred embodiment of the present invention, the stored electric charge in the utilized condenser of condensers 9 is preselected to be exactly proportional to the exposure ratio of time to brightness and thus the desired exposure ratio can be exactly attained automatically.

It is obvious that the present invention may be evidenced in many difierent forms and embodiments, the particular embodiments shown and explained herein are illustrative only and are not restrictive on the scope of the claims. For example, a relay may obviously be employed in place of a thyratron such as described in conjunction with the specific embodiment of FIG. 2.

I claim:

1. In a mass spectrometer wherein a specimen is disposed to be sparked by the application of an electric current to form ions that are accelerated into an ion beam and conducted to a photosensitive plate, the improvement in combination therewith comprising:

(a) a shielding member formed of an electrically conductive material and provided with an opening of predetermined dimensions, said dimensions being smaller than a cross section of said beam, said member being positioned to interrupt said beam and allow only that portion of the beam that will pass through said opening to penetrate said member, the balance of said ions impinging on said member so as to impose an electric voltage therein; and

(b) means for measuring said voltage and at a predetermined voltage interrupt said electric current.

2. Apparatus as set forth in claim 1 wherein said voltageis accumulated in a condenser and means are provided to measure the voltage buildup in said condenser and interrupt said electric current when said potential has reached said predetermined voltage.

3. Apparatus as set forth in claim 1 wherein two said shielding members are positioned in tandem within said beam, the first to receive said beam having an opening of predetermined dimensions that are greater than the predetermined dimensions of the opening of the second of said members to receive said beam and said measuring means being disposed to measure the voltage rise of the said second member to receive said beam.

4. Apparatus as set forth in claim 3 wherein sad first member to receive said beam is grounded and means are provided to determine the current flow from said mem ber causd by ion impingement thereon.

5. The method of providing a predetermined exposure of ion beam impingement on a photosensitive plate in mass spectroscopy, where said beam is of predetermined dimensions and the desired exposure time is dependent on the brightness of the beam comprising:

(a) positioning a shielding member formed of an electrically conductive material and provided with an opening of predetermined dimensions within said beam so as to allow only that portion of the beam that will pass through said opening to penetrate said member and so that the balance of said ions will impinge on said member and impose an electric voltage thereon; and

(b) measuring said voltage and interrupting the ion source when said voltage reaches a predetermined level, said level corresponding to a preselected exposure of said plate to a given ion brightness.

6. The method of providing a predetermined exposure of ion beam impingement on a photosensitive plate in mass spectroscopy, where said beam is of predetermined dimensions and the desired exposure time is dependent on the brightness of the beam comprising:

(a) positioning two shielding members formed of electrically conductive materials and each formed with openings of predetermined dimensions in tandem in the path of said ion beam so that only the portion of said beam that will pass through said openings penetrate said members and the balance of said ions impinging on said members transmit their electrical charges thereto, said opening of the second of said members to receive said beam being smaller than the opening of the first of said members to receive said beam so that separate fractions of the ions of said beam give up their charges to each said members;

(b) grounding said first member to receive said beam and determining the current flow from said member; and

(c) measuring the voltage increase of the second member to receive said beam and interrupting the ion source When said voltage reaches a predetermined level, said level corresponding to a preselected exposure of said plate to a given ion brightness.

7. In a mass spectrometer wherein a specimen is mounted as electrodes and by the application of an electrical current ions are formed by a spark occurring therebetween which are accelerated into an ion beam and conducted to a photosensitive plate, the improvement in combination therewith comprising:

(a) an oscillator circuit disposed to provide high frequency current to said electrodes when provided with triggering signals;

(a) an astable multivibrator circuit disposed to provide triggering signals to said oscillator circuit;

(0) two electrically conductive shielding members positioned in tandem within said beam the first to receive said beam having a slit opening of predetermined dimensions that are greater than the predetermined slit openings of the second of said members to receive said beam, both said openings being of lesser dimensions than said beam;

((1) means for grounding the first of said members and means for measuring the electric current flow from said member caused by ion impingement thereon;

(e) means for measuring the voltage of the second said members; and

(f) means for interrupting the pulses emitted by said 5 multivibrator circuit when said voltage rises to a preselected level.

8. Apparatus as set forth in claim 7 wherein the elements of said multivibrator circuit are electron tubes and a conductor conducts said voltage to a capacitor and means are provided to change the charge of the grid of one of said tubes so as to interrupt said vibrator and shut ofi' said current flow to said electrodes when said voltage in said capacitor reaches a predetermined level.

9. Apparatus as set forth in claim 8 wherein an elec- UNITED STATES PATENTS 6/ 1964 Gutter 250-495 4/1966 Herzog et a1 250-41.9

RALPH G. NILSON, Primary Examiner.

trometer is disposed to energize a thyratron when said 10 A. L. BIRCH, Assistant Examiner. 

